(734g) Systematic Ligand Design for Methane to Methanol Transformation: A Computational Approach

The selective functionalization of inert hydrocarbons has attracted major interest over the past decade. Terminal metal-oxygen bonds (MO) have been a common unit for many heterogeneous and molecular catalysts that insert oxygen in inert CH bonds. Here we present a systematic optimization of metal-oxide molecular catalysts for the conversion of methane to methanol. Using high-level theoretical chemistry calculations, we show that strong-field ligands stabilize the oxo form of MO (oxidative addition mechanism), and that weak-field ligands the more reactive oxyl form (radical mechanism). For example, we show that ammonia ligands deactivate the FeO²⁺ unit more than water, and that halogens can activate the inert early transition metal oxide units, such as NbO and ZrO. Specifically, the high spin oxyl states of NbO and ZrO offer lower energy barriers for both the activation of C-H and the subsequent formation of methanol. These oxyl states are stabilized over the lower energy (in case of naked ZrO) oxo-states by the attachment of halogens to the metal. Our results suggest practical rules of how to adjust the balance of oxo/oxyl character and reactivity of metal-oxygen units. More metals will be employed in the near future and the connection of the oxo/oxyl character to selective C-H activation will be investigated.